TABLE OF CONTENTS Many missions require a high-impulse velocity change burn as well as pulse mode attitude control operation. For example, a planetary orbiter requires a high-impulse single burn for orbit insertion and pulse mode for attitude control. Geosynchronous spacecraft also require a high-impulse burn to establish the required orbit as well as pulsing performance for ACS. Magellan used a solid motor for orbit insertion and a monopropellant system for pulse mode and small translation burns. Galileo used a bipropellant system for high-impulse burns and for pulse mode operation. Viking Orbiter used a MMH/N 2O 4 bipropellant system for orbit insertion and trajectory correction maneuvers and a cold-gas system for attitude control. Geosynchronous orbiters commonly use a solid motor for orbit insertion and a monopropellant system for ACS. There is a new technology that promises a better solution by using hydrazine as bipropellant fuel, replacing monomethyl hydrazine, and also as a monopropellant for pulsing. This is called the dual-mode system. The system is shown schematically in Fig. 4.44.
The advantages of the dual-mode system are 1) the ability to use the hydrazine as a monopropellant in attitude control thrusters and as the fuel in bipropellant main engines with resulting system simplification, and 2) a significant increase in I sp for the orbit insertion burn. A typical solid motor delivers an I sp of 290 s; the dual-mode bipropellant system delivers about 317 s. The dual-mode system is used on the...
